Many different types of refrigeration units are available. As is well known in the art, in a refrigeration unit, a compressor pumps a refrigerant in a circuit to a condenser, then through an expansion valve, next to an evaporator, and then back to the compressor. Different types of evaporators are known in the art. One of the more commonly used types is the direct expansion evaporator.
Typically, lubrication oil is required to be used in the compressor. Invariably, lubrication oil from the compressor becomes mixed with the refrigerant as the refrigerant passes through the compressor. The result is that a refrigerant/oil mixture circulates through the circuit. It has been recognized that, in certain types of refrigeration units, the oil has tended to separate from the refrigerant and to accumulate in the evaporator. It is also well known that the accumulation of oil in the evaporator adversely affects the efficiency of the entire refrigeration unit. For these types of evaporators, various methods have been proposed for removal of the accumulated oil from the evaporator. For example, U.S. Pat. No. 3,782,131 (Merryfull) discloses a refrigeration unit which includes a flooded evaporator with a surge drum(s). Oil which accumulates in a lower portion of the drum is removed through an oil pick-up tube (35). However, in a “flooded” evaporator, the rate at which the refrigerant/oil mixture flows through the evaporator (and exits therefrom) is generally much lower than the flow rate of the refrigerant/oil mixture upon exiting a direct expansion evaporator.
In the prior art, it is generally thought that the rate at which the refrigerant/oil mixture flows through a header of the direct expansion evaporator (i.e., when exiting the direct expansion evaporator) is sufficient to prevent any significant accumulation of oil in the evaporator. This is because, in a direct expansion evaporator, substantially all of the refrigerant/oil mixture is vaporized at the outlet of each evaporator tube, i.e., at the intersection of each evaporator tube with the header. After discharge into the header, the refrigerant/oil mixture moves through the header and exits from the header (i.e., exits from the direct expansion evaporator) into a suction tube, which leads from the evaporator to the compressor.
Accordingly, it has generally been thought that the vaporization of substantially all of the refrigerant/oil mixture at each evaporator tube outlet in a direct expansion evaporator results in an increase in velocity of the mixture which is sufficient to prevent accumulation of oil in the evaporator. For instance, the following excerpt is from the ASHRAE Refrigeration Handbook (2002) (pp. 2.16–2.17):                . . . flooded evaporators can promote oil contamination of the evaporator charge because they may only return dry refrigerant vapor back to system.        . . . in general, direct-expansion . . . system evaporators have fewer oil return problems than do flooded system evaporators because refrigerant flows continuously at velocities high enough to sweep oil from the evaporator.        
However, contrary to the generally accepted view, it has been determined that oil does accumulate in a direct expansion evaporator, as will be described.
There is therefore a need for an extraction apparatus for extracting oil which has accumulated in a direct expansion evaporator.